The isolated rat heart will be used as a model organ system for the study of long term organ preservation by the technique of controlled hypothermia. Prior to cooling, various cryophylactic agents (CPA) will be administered to excised rat hearts at controlled rates by means of a linear CPA administration device with CPA local concentrations determined by a specially developed probe. Sensitive viability assays will be used to determine the effects of CPA, its concentration, and the rate of addition on heart viability. Thermal protocols, including freezing, will be effected by means of a dedicated thermal control system which provides cooling and warming at constant rates in the range (0.3 degrees centigrade/min to 10 degrees centigrade/min at temperatures between 40 degrees centigrade and minus 40 degrees centigrade. Both protected and unprotected hearts will be employed to determine the effects of cooling rate, warming rate, and cryophylactic agent parameters on viability.